Film thickness control for electrostatic coating systems



Sept. 24, 1968 E. KOCK 3,402,697

FILM THICKNESS CONTROL FOR ELECTROSTATIC COATING SYSTEMS,

Original Filed March 13, 1964 FILM THICKNES:

INVENTOR: .Z'RHAHD Km :14:

MTQW United States Patent 3,402,697 FILM THICKNESS CONTROL FQR ELECTRO-STATIC COATING SYSTEMS Erhard Kock, Toledo, Ohio, assignor to TheDeVilbiss Company, Toledo, Ohio, a corporation of Ohio Continuation ofapplication Ser. No. 351,666, Mar. 13,

1964. This application Feb. 23, 1967, Ser. No. 618,237

2 Claims. (Cl. 118624) ABSTRACT OF THE DISCLOSURE An electrostaticcoating apparatus for controlling the thickness and uniformity of thecoating deposited on a workpiece. A linear array of a plurality ofuniformly charged electrostatic spray guns are mounted on a reciprocablestem and an electrode is provided on said stem at each side of saidarray, spaced from the adjacent gun at a distance substantially equal tothe spacing between adjacent spray guns and charged to the samepotential, the ends of the electrodes being spaced from the work atsubstantially the same distance as the discharge ends of said sprayguns.

This is a continuation of application Ser. No. 351,666, filed Mar. 13,1964.

This invention relates to a film thickness control for electrostaticcoating systems and has for its primary obcoating an extensiveworkpiece, as for example, a long panel suspended from an overhead,horizontally moving conveyor, it has been found that it is necessary touse a plurality of spray guns in an array. This expedient is adoptedbecause the rate of material flow through a single electrostatic spraygun is somewhat limited for highest efficiency and if it were attemptedto cover the entire workpiece with one gun, it would be necessary tooperate the conveyor at an intolerably low rate of travel, and toreciprocate the gun vertically through a stroke substantially equal tothe height of the workpiece. When an array of a plurality ofelectrostatic spray guns is used, more material can be deposited in aunit of time and the conveyor speed can be increased to a satisfactory,conventional value.

The association of a plurality of electrostatic spray guns in an array,however, leads to problems in obtaining uniformity of the filmthickness, or the thickness of material applied to thejworkpiece,whether or not the array is reciprocated. If the array is reciprocated,more uniform deposition can be obtained than without the movement. Thepresent invention relates to a means to control the film thicknessdeposited by an array of electrostatic spray guns and finds its highestuse where the array is reciprocated, but will also be found to improvethe uniformity of the deposition from even a stationary array.

It is desirable that the variation in film thickness, or the thicknessof a deposited coating of paint, for example, be quite uniform Over theentire coated area. The film thickness uniformity is expressed as apercentage or ratio between the thickest and thinnest parts of thecoated areas. For example, if the thickest coated area is measured andfound to have a coating of 0.010 in thickness, and thinnest area ismeasured and found to have a deposited coating of 0.0075", theuniformity is said to be 75 percent. In practice, 70 percent uniformityis considered to be acceptable, and a uniformity of 80 percent isexcellent.

In electrostatic coating systems, particularly where a high charge isimparted to the coating material by a very intense electrostatic fieldset up adjacent the issuing orifice, the spray pattern of a spray gun isinfluenced and may be drastically affected by the presence ofneighboring similarly charged apparatus. For example, if a single spraygun has a round deposition pattern, the association therewith of a likegun will cause a distortion of the pattern of each gun into anon-circular configuration that trails oil? into a sparse spray on theside removed from the neighboring gun, and that is somewhat more densein the areas between the spray guns. The resulting combined pattern hasa pronounced tendency to be thicker in the central areas and thinner atthe outside areas to the point where the coating thickness uniformitywill be measured at less than the desired value.

When three or more electrostatically charged spray guns are associatedin a linear array it will be found that the pattern of the center gunsis noticeably compressed while the patterns of the outside guns in thearray are undesirably extended away from the center to give anon-uniform pattern that is difiicult to work with and that may producean unacceptable coating on a large panel, for example. The reason forthis difficulty apparently lies in the fact that with three or moreguns, the inside spray patterns are subject to electrostaticinterference from two sides, while the outside spray patterns areaffected only from one side. This results in inside spray patterns thatare electrostatically more compressed than the outside patterns, thusproducing interior patterns of either a greater maximum film thicknessor patterns having a steeper slope or both.

The present invention is based on the discovery that the combinedpatterns of a plurality of electrostatic spray guns can be made uniformto an unexpectedly high degree by introducing into the electrostaticfield at each end of the array of a plurality of spray guns, a fieldmodifying or compressing electrode charged to the same polarity as thecharging electrodes of the spray guns themselves. The field modifyingelectrodes may carry a lower or higher potential than the spray gunsthemselves, depending on the degree of pattern correction andcompression that may be desired for a given workpiece, and may beenergized from the same power source as the spray guns themselves, orfrom a different power source. In the most preferred embodiment of theinvention, the spacing of electrostatically charged guns within thearray is uniform, the guns are charged to the same potential, and thespacing between the end guns and the field modifying electrodes issubstantially equal to the spacing between guns, the electrodes beingcharged to the same potential and the same polarity as the gunsthemselves. In this manner, the compressing effect on the spraydeposition pattern of the gun adjacent the electrode will besubstantially the same as the compressing effect on said patternproduced by the neighboring spray gun in the array.

A preferred embodiment of the invention is illustrated diagrammaticallyin the attached drawings forming a part of this specification and inwhich:

FIG. 1 is a diagrammatic view of an array of electrostatic spray gunsembodying the present invention;

PIG. 2 is a graph of film thickness or film cross section of depositedcoatings from an array of three electrostatically charged spray gunswithout the modifying electrodes provided by the present invention; and

FIG. 3 is a graph similar to FIG. 2 indicating the film cross sectionresulting from the present invention.

In the drawings, 10a, 10b and designate electrostatically charged sprayguns of the type in which the spray gun itself, or an electrode carriedby the spray gun, is brought to a high potential, for example 60 kv. tokv., preferably negative with respect to ground. Particles of coatingmaterial issuing from the gun acquire a high negative charge and aredeposited on a grounded workpiece W under the influence of theelectrostatic field which exists between the gun and the work. Ifdesired, a positive charge may be placed on the gun and thus on theparticles and further, the work may be at other than ground potential asis well known in the art. It will be assumed for purposes of thisdescription that the spray pattern of a single spray gun is essentiallycircular. That is, if the gun is not moved, and if the work W is a largeflat panel held stationary, coating material will be deposited on thework over :a circular area. Because the density of the coating it notcompletely uniform throughout the circular area, it is known in theart'that the spray gun may be reciprocated to deposit a band of coatingmaterial that may be acceptably uniform in cross section. Further, ifthe work W is moved laterally on a conveyor across the space in front ofthe gun while the gun is reciprocating vertically the entire panel willbecome coated with a layer of material that should be essentiallyuniform in thickness. The rate of reciprocation and the rate of lateraltraverse are so related that the gun makes several vertical passesacross the work during the time taken for the conveyor to traverse adistance equal to the width of the pattern.

Electrostatic spray guns are limited in the quantity of coating materialthat can be satisfactorily atomized, charged and deposited in any givenperiod of time. This quantity of material and the area of work to becoated determine the speed at which a conveyor carrying the work can beoperated. For this reason, among others, it is frequently necessary toemploy an array of two or more electrostatically charged guns. Thepresent invention is applicable to any array, but is particularly usefulWhere three or more guns are associated as a linearly arranged group.The benefits of the invention will be most pronounced when the array ofguns is reciprocated so that the combined spray sweeps across the workin a direction transverse to the direction of conveyor movement.

As shown in the drawings the spray guns 10a, 10b and 100 are mounted inany suitable manner on a stem 11. The spacing d between guns can beassumed to be adjustable for purposes which will become apparent. Thestem 11 is supported at its top and bottom by insulating arms 12 and 13which are pivoted to the steam and to a common frame 14. One of the arms13 is extended rearwardly of the frame 14 and is driven to itsreciprocating motion by an eccentric 15 powered by a motor 16 andconnected to the arm 13 by a connecting rod 17.

The spray guns 10a, 10b and 100 may be of any suitable type since thepresent invention is applicable to any electrostatic coating systemwhether the coating material is atomized by mechanical or electrostaticforce. The diagrammatic showing in FIG. 1 indicates guns of the type inwhich liquid coating material is supplied to each gun through a flexiblehose or tube 18 and air is supplied through a flexible hose or tube 19and is used to atomize the liquid and to operate the on-oif controlcylinder within the gun itself. Air operated and air atomizing sprayguns are, of course, well known in the art. In the diagrammaticdisclosure, the guns themselves are charged from a power pack E, oneside of which is grounded and the opposite terminal of which carries anegative charging voltage, -V. If desired, the electrostatic field fromeach gun may be intensified by the introduction of a central,needle-like electrode disposed in or closely adjacent to the nozzle ofthe gun where air atomization first takes place.

When the spray guns 10a, 10b and 100 are charged simultaneously, theelectrostatic field originating in each gun has a decided distortingeffect on the field of its neighbors. In the electrostatic coatingprocess the field intensity is preferably very high in the areas wherethe coating material, if it is liquid, is broken up into droplets oratomized. The atomized particles will thus acquire, by ionicbombardment, the high charge that is requisite for efiicient depositionof the atomized particles on the work. In passing to the work, theparticles are largely influenced by the shape and intensity of theelectrostatic field in the region between the guns and the workpiece,and if the field is distorted at the ends of the array the deposition onthe work will be unequal in thickness in the direction of the length ofthe composite pattern from the three spray guns. FIG. 2 showsdiagrammatically the inequality of the film thickness to be expected. Inthis figure the abscissa represents film thickness, greatly expanded,and the ordinate represents lineal distance from end to end of the arrayof guns or a vertical dimension in the orientation of the parts shown inFIG. 1. The results are those obtained from vertically reciprocating anarray of three spray guns in front of a moving workpiece suspended on aconveyor and moving laterally with respect to the direction of sprayprojection. The three solid line curves 20, 21 and 22 show thecontribution to the total film thickness made by each of theelectrostatic spray guns charged to like potential, uniformly spaced andhaving the same quantity of coating material supplied to each. Thus theareas under the three curves are equal because they represent equalvolumes of deposited material. The dash line 23 indicates the total filmthickness and is the arithmetic sum of curves 20, 21 and 22. The curves20, 21 and 22 are taken with the stem 11 reciprocating through a pathhaving a maximum lineal dimension approximately equal to the spacingdistance between guns. It has been found that this spacing andreciprocating stroke length are essentially optimum although variationstherein may be tolerated and may, in fact, be dictated by thecharacteristics of the work to be coated.

The film thickness variation shown in FIG. 2 would probably not becommercially acceptable, even though the efliciency of the deposition ofthe entire electrostatic coating system may be quite high. Too much ofthe liquid coating material appears in the center area of the work paneland too little is deposited in the outer areas. The center line of thecoated work-piece is designated C in FIG. 2.

The present invention makes it possible to provide a multiple spray gunarray in which the uniformity of deposition is greatly improved. I havefound that if the linear array of spray guns 10a, 10b and 10c isterminated at each end by a field-modifying electrode 30, each gun maybe made to have a similar pattern, and the film thickness uniformity ofthe deposition from the array may be made to exceed percent. That is, noarea of the work panel W will have a deposited film that is less than 80percent of the maximum thickness of the film on any portion of thepanel. Each electrode 30 may comprise, for example, a simple rod-likeelement having a sharp point. The electrode should be such that it willexert the same field influence as one of the spray guns in the array.

The location of the field modifying electrodes 30 may be varied, but inthe preferred execution of the invention, the electrode geometry andspacing is such that the electrostatic field emanating from theelectrode 30 influences or compresses the pattern of the adjacentelectrostatic spray gun in substantially the same manner and tosubstantially the same extent as the pattern is influenced by the fieldof the neighboring gun in the array. To this end, if a given spacing dis used between spray guns 10a and 10b, and between spray guns 10b and100, the same spacing d would be utilized between spray gun 10a and theup per field modifying electrode 30, and the same spacing d would beused between the lower spray gun 10c and the lower of the fieldmodifying electrodes 30. Similarly, the spacing between the end of theelectrode and the work W would be the same as the spacing between thecharging electrode of the spray guns and the work. It is, of course,apparent that the electrodes 30 may, therefore, be most easily chargedfrom the power pack E to the voltage V to which the spray guns arecharged. If, for any reason, it is necessary to apply a higher or lowercharging voltage to the electrodes 30, it may be necessary to use aseparate power pack for this purpose. For example, if the work W is alarge concave panel so that the distance between the work and the fieldmodifying electrodes 30 is somewhat less than the distance between thespray guns themselves and the median plane of the panel W, the voltageon the field modifying electrodes 30 may be reduced in view of the lowerelectrode-work spacing that occurs by reason of the configuration of thework.

FIG. 3 diagrammatically indicates the effect of the presence of thefield modifying electrodes 30. It will be seen that each of the sprayguns a, 10b and 10c in this instance, when reciprocated, depositscoating material in a similar pattern and the film thickness of thespray from each of the three guns is designated 31, 32 and 33. Thecontribution to the film on the work of each of the spray guns isindicated as essentially the same, and the sum of the three separatefilms is essentially a flat, straight line as indicated by the dashedline 34, which is the arithmetic sum of the increments 31, 32 and 33deposited by the separate spray guns. In this figure, C designates thecenter of the coated area of the work.

Film thickness uniformities upwards of 80 percent have been obtainedwith the physical arrangement of the elements approximately as shown inFIG. 1. Typical parameters for the operation of a system including threespray guns as there shown would be as follows:

Spacing between guns in 10 Spacing between modifying electrodes andadjacent guns in 10 Spacing between guns (and electrodes) and workpiecein Charging voltage kV 12() Reciprocator stroke length in 8 /2Reciprocator frequency c./min 60 Conveyor speed ft./min 15 Materialdischarge c./min./ gun 150 Material discharged (1) 1 Baking enamel;viscosity 21 sec. Zann No. 2 cup.

In an electrostatic system, it is recognized that the overlap of thepattern from the spray guns with respect to the edge of the article tobe coated should be controlled. If the overlap is too great, paint willbe lost by spraying beyond the article, and if the edge is sharp, thereis a tendency for the paint to build up to an excessive thickness at thesharp corner. Therefore, the end gun at the top and bottom of the arrayshould be spaced inwardly towards the center of the work panel when itstands at the mid-point of its reciprocating stroke, by a distance aboutequal to one-half of the stroke length.

The effect of variations in spacing between the spray guns and the workpanel has been investigated, and it has been found that, while somewhatlower film thickness uniformities are obtained as the spacing isincreased beyond the optimum, the system is relatively insensitivebetween 15" and 21" with charging voltages of -120 kv., and harmonicreciprocation of the stem 11 through strokes that were adjustablebetween about 8 and about 10", the reciprocation frequency being cyclesper minute. Where higher surface speeds produced by greater conveyorspeeds or rotation of the work were used, it was found thatreciprocating frequencies in the order of cycles per minute or more aredesirable.

While the invention has been disclosed in conjunction with a specificform and disposition of the parts, it should be expressly understoodthat numerous modifications and changes will suggest themselves to thoseskilled in the art, and that such changes may be made without departingfrom the scope of the appended claims.

What I claim is:

1. Electrostatic coating apparatus comprising a reciprocable stem, alinear array of a plurality of uniformly spaced uniformly chargedelectrostatic spray guns mounted on said stern and having theirdischarge ends substantially uniformly spaced from the median plane ofthe work to -be coated, and an electrode mounted on said stem at eachend of said array spaced from the adjacent gun a distance substantiallyequal to the spacing between adjacent spray guns and charged in the samepolarity as said adjacent gun and to the same potential, the ends ofsaid electrodes being spaced from the work at substantially the samedistance as the discharge ends of said spray guns, whereby each of saidelectrodes is effective to influence the deposition pattern of itsadjacent gun in substantially the same manner and to substantially thesame extent as said pattern is influenced by the presence of theneighboring gun in said array, and means to reciprocate said stem andthe array of spray guns and electrodes carried thereby.

2. Electrostatic coating apparatus in accordance with claim 1 in whichsaid last means operates to reciprocate said stem and said array ofspray guns and electrodes through a distance substantially equal to thespacing distance between adjacent guns.

References Cited UNITED STATES PATENTS 2,658,009 11/ 3 Ransburg 239-3 XR2,733,171 1/1956 Ransburg 118631XR 2,869,510 1/1959 Renner 1186262,877,137 3/1959 Juvinall et al 117-93.44

FOREIGN PATENTS 1,156,341 10/1963 Germany.

PETER FELDMAN, Primary Examiner.

